Selective Circuit Breaker

Individual circuit protection for factory systems

Classes in basic electricity usually include discussions on the difference between “parallel” and “series” circuits. The example teachers will cite is a set of Christmas lights — how with the old-fashioned series lights, all the bulbs would go dark should one fail. From such a clear example, one would have expected that all the students would know not to design any system in series.

But there are always exceptions.

Many manufacturing lines and processes are employing 24 VDC power as a standard for sensors, actuators, field bus couplers and controls, with the electricity supplied by switched-mode devices. Unfortunately, when an overload occurs in a single circuit of the switched supplies, the supply responds by shutting down output to all circuits. In many factories, that situation effectively stops all production until the fault is ascertained and repaired, a downtime that can translate into thousands of dollars per minute. Moreover, power supplies trying to recover from an overload often create a rapid cycling of on-off output, sometimes referred to as “hiccupping.” When this condition occurs, the power supply cannot provide sufficient voltage — something more than 18V — for electronic components to operate. Equipment powered by hiccupping power supplies can cause worker injury and damage product.

To counter this problem, E-T-A Circuit Breakers, Mt. Prospect, IL, has introduced a circuit breaker system that can selectively protect individual components powered by 24 VDC switched power supplies. This solution limits the scope of the disruption and allows the operator to quickly determine which circuit is at fault. Through the application of two technologies — a mechanical circuit breaker and an elaborate electronic switch — the ESS20 provides physical isolation of the load circuit during an overload disconnection, or when the manual ON/OFF button is pressed, unlike circuit protection used in power supplies.

For example, a typical 6A thermal-magnetic circuit breaker trips within 10 seconds at 3 times rating. The problem is that a power supply would receive up to 18 amps for as long as 10 seconds, enough to cause the power supply to shut down. A 6A ESS20 also acts like a conventional circuit breaker, breaking the circuit when the current is too high for too long, but it also limits the max current to 10.8 amps instead of 18 amps. How does it do this? It contains an electronic switch that calculates the current that could flow, then adds impedence to the circuit to reduce current to a safe value. Thus, the ESS20 limits the maximum current to no more than 1.8 times rating, either 5.4 or 10.8 amps.

Standard thermal-magnetic breakers lack this ability, requiring a higher-than-ideal current rating to avoid nuisance tripping. The unit also employs active current-limiting technology, which offers greater equipment protection while allowing a breaker of lower current rating to be specified. A thermal bimetal disc adds failsafe protection by tripping the breaker in cases of electronic failure. At-fault status is indicated by an LED, and the breaker unit can additionally send a signal to a logic controller or factory software so that the line or process can be safely shut down when an overload occurs.

With a width of only 12.5 mm, the ESS20 unit snap-mounts in the company’s Module 17plus power distribution system that, in turn, can accommodate two ESS20’s. Any number of modules may be installed side-by-side on a DIN-rail and connected via bus bar to create a custom rack. All electrical connections are made by means of spring-loaded terminals.

—RM